iStock/Thinkstock(PHILADELPHIA) — Authorities are investigating a string of suspicious explosions in Pennsylvania, according to ABC Philadelphia station WPVI-TV.

The most recent explosion occurred Thursday afternoon, when a worker cutting grass hit the suspicious device with a lawnmower, WPVI-TV reported. It left a 2-foot crater in the ground, and both FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives agents are investigating.

The worker was uninjured.

Resident Melissa Pfinster was with her kids at home when she felt the explosion, which was so forceful it knocked over a bookcase, she told the station.

“It’s kind of scary when you got little guys,” Pfinster said.

There have been upwards of three dozen explosions over the past two months in upper Bucks County and Lehigh County, according to WPVI-TV.

Richland Township Police Chief Richard Ficco said the reports about the devices have typically come in overnight before dawn.

“The ground shifting, almost shaking,” Ficco told WPVI-TV in a May interview, describing some of the calls his and other area departments have received in the last several weeks. “The buildings are shaking, ceiling tiles are moving, and windows are rattling,”

“Definitely disconcerting,” he added. “I would say unnerving to some people.”

An explosion a couple of weeks ago left a 10-foot-wide crater that was a foot deep. No one has been injured, and no suspects have been named.

Authorities warned those who live in the area not to walk on uncut grass on the side of the road and also not to mow the lawn in case there are other explosive devices.